This project tested the use of 3-dimensional multi-component seismic (MCS) data recorded with 4-component ocean bottom cables (OBC) as a method for improving the industry's ability to detect and characterize gas hydrates, assess the gas hydrate resource, and predict the stability of hydrate-containing sediments.

Impact

The methodologies developed by this project can serve as a road map for interpreters attempting to avoid shallow hazard zones in the northern Gulf of Mexico, and identify potential areas for commercial exploitation of gas hydrates.

Accomplishments (most recent listed first)

Acquired processed 3-dimensional (3-D) MCS data from a multi-client survey over 46 OCS blocks of East Cameron South area recorded in 1999-2000 (via industry partners Seitel Data and WesternGeco),

Performed 3-D interpretation (software provided by Landmark Graphics Corp.), and

Developed 3-D interpretation-based methodology and published final report.

P-wave image with its depth-equivalent C-wave image highlighted

A multi-component system is superior to a single-component seismic system for evaluating gas hydrate reservoirs and their surrounding environment because the depositional system can be imaged with both compressional waves (sound waves that pulse through the bottom sediments, called P-waves) and shear seismic waves (waves that move at various crossing angles, called S-waves). Integrating seismic wave types can provide more information about sediment relationships, rock-type distributions, and pore-filler material. In addition, the P-wave velocity (Vp) and S-wave velocity (Vs) can be used to calculate elastic constants and to infer the shear modulus and mechanical strength of the sediments, important factors relating to seafloor stability. In general, a more detailed characterization of marine gas hydrate systems can be obtained if gas hydrate target intervals are described in terms of integrated P and S seismic data rather than P-wave data alone, as is typical in marine seismic investigations.

The principal barrier to overcome when imaging marine gas hydrate sediments, with both P and S-waves, is that S waves do not propagate in fluids. This problem may be overcome if four-component ocean-bottom sensor cable (4-C OBC) is utilized to acquire seismic data at the sea floor.

The initial objectives of the project were to see if P-wave reflection quality could be improved by combining hydrophone and vertical-geophone data, improving structural interpretation in areas with P-wave data "wipeouts". Additional objectives were to evaluate Vp/Vs velocity ratios and seismically derived Poisson's ratios for characterizing near-seabed sediments and to develop practical post-stack 3-D data interpretation methodologies that capitalize on the unique characteristics of the multi-component OBC data.

Poisson’s ratio for layer between seafloor and stratal surface

The research concluded that developing robust 3D interpretation-based methodologies would in fact allow geoscientists to exploit the unique characteristics of multi-component data. Time-dependent attributes like Vp/Vs and Poisson's ratios strongly depend on accurate depth registration between seismic events. Depth registration of wave images should rely on correlating map-based images (either time slices or horizon slices), and correlation between section (2D) views should be done sparingly. Quality control mechanisms are vital to ensure robust correlation and to produce valid MCS-based attributes for shallow marine characterization.

In addition to the information provided here, a full listing of project related publications and presentations as well as a listing of funded students can be found in the Methane Hydrate Program Bibliography [PDF].